The major factor which limits the size of the exposed field in lithographic processing of a semiconductor wafer is the accuracy with which the wafer and a mask may be aligned. Since mask and wafer distortions increase with field size, alignment accuracy increases with decreasing field size. For this reason, sub-field optical and X-ray step-and-repeat exposure systems are commonly used in modern lithographic processing Unfortunately, both system complexity and die cost increase, and throughput decreases, as the number of sub-fields per wafer is increased. Thus, it is desirable to maximize the field size at a given level of alignment accuracy.
Even though alignment accuracy in modern X-ray lithographic systems has increased for relatively large field sizes, it is often impossible to achieve adequate alignment accuracy over the full wafer surface and sub-field exposure becomes necessary Step-and-repeat systems are used in optical lithography to step the wafer in an X- or Y-direction between exposures so that a relatively small field is repeatedly exposed on the wafer surface. Step-and-repeat systems typically require complex movement and alignment devices to accurately repeat the exposure pattern across the surface of the wafer.
In accordance with an illustrated preferred embodiment of the present invention, a rotate-and-repeat apparatus makes effective use of the increased size of the distortion-free field in modern X-ray lithographic systems. A semiconductor wafer is divided into four quadrants and a mask, having an active diameter which is smaller than that of the wafer, is aligned over one quadrant. After the first wafer quadrant is exposed through the mask the wafer is rotated in successive 90 degree increments and each of the other three quadrants is exposed through the mask. The use of rotational movement of the wafer, rather than linear movement, allows the rotate-and-repeat apparatus to be smaller and less complex than a corresponding step-and-repeat apparatus would be. Alignment of the mask and the wafer in the rotate-and-repeat apparatus may be achieved with capacitive aligners as shown in the copending Neukermans et al U.S. patent application Ser. No. 541,385, which is incorporated herein by reference.
In accordance with another illustrated preferred embodiment of the present invention, a rotatable diaphragm is positioned between a radiation source and the mask. The wafer, mask and diaphragm are all of substantially the same diameter and are all divided into quadrants. Use of one or more diaphragm transmissive quadrants and rotation of the diaphragm and wafer allows selective exposure of the wafer quadrants.